The corn kernel is comprised of a number of components, each being best suited for various commercial use patterns. The fibrous hard outer shell is the pericarp, commonly called the bran. The interior of the corn kernel consists of the endosperm and the germ. The germ contains a much higher percentage of protein compared to the other parts of the kernel and is the primary source of corn oil. The portion of the corn kernel which adheres to the corn cob is commonly referred to as the tip cap.
Wet corn milling processes can separate corn into the various major components of the kernel. In a wet milling process, the corn is firstly steeped in an aqueous solution to soften the kernel and then is ground and further processed to free the germ. Current technology is such that in a dry milling process, for the production of ethanol, the corn kernel is ground and is fed directly into the ethanol process with no separation of the corn kernel achieved. 100% of whole corn kernel is fed through the process in the production of ethanol.
The degree of separation of germ from endosperm that is achieved with conventional degerminating machines can cause problems in the overall milling process. For example, in certain commercial degerminators, the grain kernels are rubbed more against one another than against the metal of the machine. As a consequence, even though relatively good separation of some of the germ is achieved, a large quantity of fine material is generated which often is high in fat content since they contain fine particles of pulverized germ.
In certain prior art, the corn kernels are cleaned to remove foreign material and then fed into cracking rolls, which are typically used by dry millers to reduce the size of corn kernels. The cracked corn is cleaned (screened or sized) then certain fractions are put through another set of rollers that reduce the size again before re-cleaning. This process is very much like flour milling, and like flour milling it produces a selection of different products. Each product has varying levels of oil, starch, protein and fiber.
Ponnampalam et al (Applied Biochemistry and Biotechnology 115: 837-842) demonstrated that the removal of germ and fiber from whole corn kernels can improve the efficiency of ethanol production. The “germ and fiber removed corn” for this test was prepared by blending corn grits product together with degermed corn meal. However, this reference concludes that ethanol production from fermentation of corn, with the germ and fiber removed, offers efficiencies and would likely be economically beneficial but provides no recommendations as to how to accomplish this on a commercial scale.
Generally speaking the corn kernel is comprised of three major components, these are: 1. Endosperm—which is comprised primarily of starch. 2. Germ—which contains the bulk of the extractable corn oil and, 3. Bran—comprised mainly of the high fibre pericarp and tip cap. For the efficient production, ethanol plants require primarily starch and a small amount of protein.
There is an approximate 40% density difference between corn germ and endosperm and an approximate 75% difference in density between the bran (pericarp/tip) and germ. It is notable that as product size is reduced, the density will also change. Finely ground pericarp and tips will increase in density significantly after fine grinding. Grinding the pericarp and tips too fine results in a poor separation.